Recovery of dilute caustic soda solutions from spent liquors containing hemicellulose

ABSTRACT

Caustic spent liquor, from the processing of pulp material such as wood and cotton pulp in the purification of cellulose, may contain up to 10 percent caustic soda along with significant amounts of both inorganic salts and organic matter made up of hemicellulose, pentosans, hexosans, galactans, and the like, which are extracted from the pulp by the action of the caustic solution.

United States Patent Kennedy 14 1 1am. 18, 1972 [54] RECOVERY OF DILUTE(IAUSTIC 1,606,338 1 1/1926 Bradley et a1 ..23/49 SODA SOLUTIONS FROMSPE 2,337,262 12/1943 McOmie et al. ....23/185 X LIQUORS CONTAINING NT2,618,610 11/1952 Thomsen 162/36 HEMHCELLULOSE Primary Examiner0scar R.Vertiz Assistant Examiner-G. Alvaro [72] Inventor Davld Kennedy ClayAttorney-Birgit E. Morris and Gerard P. Rooney [73] Assignee: AlliedChemical Corporation, New York,

N.Y. [57] ABSTRACT [22] Filed: June 11, 1969 Caustic spent liquor, fromthe processing of pulp material such as wood and cotton pulp in thepurification of cellulose, [21] Appl 832234 may contain up to 10 percentcaustic soda along with signifi cant amounts of both inorganic salts andorganic matter made [52] US. Cl ..23/1s5, 23/49, 23/184, p f h mi ll l pm hexosans, galactans, and h 162/29, 162/38 like, which are extractedfrom the pulp by the action of the [51 1 Int. Cl. ..C01d 1/00, D21c11/00 caustic solution. [58] Field of Search ..23/185, 184, 49; 162/29,36

7 Claims, 1 Drawing Figure [56] References Cited UNITED STATES PATENTS1,005,882 10/1911 Rinman ..23/49 .VENT

GAS SLAKED LIME CARBONATING TOWER CARBONATED LIQUOR CAUSTIClZERHEMICELLULOSE STORAGE SETTLING TANK gTORAGE LIME SLAKER LIME KILM CARBONDIOXIDE LIME MUDS DECANTED CAUSTIC gag/1 LIQUOR WEAK CAUSTIC STORAGE TOEVAPORATORS I- w PATENTED JAN. 8 m2 AGE/VT RECOVERY OF DILU'IE CAUSTICSODA SOLUTIONS FIIOM SPENT LIQUORS CONTAINING IIEMICELLULOSE Therecovery of the caustic soda from the spent caustic liquor containingorganic matter, has not been heretofore economically feasible because ofthe high cost of processing.

An economical unitary process for the recovery of purified reusablecaustic solution from the spent caustic liquor con taining organicmatter, is herein described. The relatively pure caustic solution isrecovered after carbonation, wherein the pH is dropped from over 12 toabout 7.5-10.5, followed by causticization of the spent liquor with anexcess of lime, as calcium oxide or calcium hydroxide, to precipitatethe bulk of impurities therefrom. The entire process is carried outwithout resorting to inordinate conditions such as excessive hightemperatures of well over 100 C. and superatmospheric pressures of theprior art methods for removing organic contaminants from spent causticliquor, which add to the expense of recovery. In organic impurities arecoprecipitated, as carbonates and bicarbonates, along with thehemicellulose and other organic matter.

BACKGROUND OF THE invention Cellulosic pulp material is normallyprocessed by treating with a 17-]8 percent caustic soda solution toextract nonalpha-cellulose materials therefrom. The extraction of theseorganic materials leads to a product containing about 94 percentalpha-cellulose. The nonalpha-cellulose organic impurities extractedfrom the pulp consist mostly of hemicellulose and minor amounts ofpentosans, hexosans, galactans, and the like. These organic impuritiesas well as inorganic matter such as sodium salts, are extracted by theprocess.

The problem of recovering reusable caustic soda solution from spentliquor containing significant quantities of organic matter by anefficient and economical method, had not been solved before thisinvention.

Because of inherent expensive operating cost and tedious procedures, theknown methods of recovery, such as dialysis, evaporation, oxidation athigh temperatures and pressures (sometimes as high as 372 C. and 2,000p.s.i.) addition of expensive precipitators, and the like have not beenfound practical.

For instance, US. Pat. No. 2,774,666, offers a process of oxidationutilizing a liquid phase oxidation reactor at temperatures betweenl50-372 C. and preferably about 225 C., and pressures of 400 p.s.i. ormore, for the conversion of organic matter to carbon dioxide and all ofthe hydrogen present, to steam. After oxidation, the liquid is treatedwith lime and the precipitated calcium oxide is separated off andkilned. This process is complicated and expensive, in virtue of theliquid phase oxidation procedure. This latter procedure is effectivelyomitted in the process of the present invention.

US. Pat. No. 2,801,264 discloses the application of a complicatedprocedure comprising repeated recycling of incomplete reaction products,during the caustic extraction of liquified cellulose, to avoid removingorganic matter by incineration.

US. Pat. No. 2,980,501 describes the concentration of spent liquor byevaporation and subsequent treatment with chlorine and heat toflocculate suspended and colloidal matter. The floc is finally skimmedoff leaving the caustic soda solution for recycling.

Nowhere in the prior art is found a method which includes a unitaryprocedure of controlled carbonation and causticizing to remove organicimpurities from spent liquors, without resorting to high temperatures ofwell over 100 C. and pressures higher than atmospheric pressure, orexpensive chemicals and equipment.

SUMMARY OF THE INVENTION By the process of the present invention, spentcaustic waste from alpha-cellulose purification are processed to removeorganic impurities, such as hemicellulose, by treating the spent liquorhaving a pH of over 12, first with a carbon dioxide containing materialto reduce the pH within the range of 7.5-l 0.5 and preferably about9-10, and secondly with lime, hereinafter defined to include eithercalcium oxide and/or calcium hydroxide, to precipitate the bulk of theorganic and other impurities and regenerate the caustic soda. Thisunitary controlled carbonation and causticizing procedure is simple andeffective, in the regeneration of caustic spent-liquorcontaining organicimpurities to caustic soda solution.

DETAILED DESCRIPTION OF Til-IE INVENTION In the practice of the presentinvention the spent caustic liquor is treated with a carbon dioxide or acarbon dioxide containing material, such as flue gas or kiln gas, toadjust the pH within the above range, and preferably to about 9.0l0.0.

The carbon dioxide may be added by simply bubbling the gas streamthrough the body of spent caustic liquor. Alternatively, more effectivecontact between the spent liquor and carbon dioxide may be achieved byflowing the liquor down through the top of a packed vessel or column,countercurrent to an upwardly rising stream of carbon dioxide, at roomtemperature and atmospheric pressure.

What is precipitated during this carbonation step is not known, but thefact remains that by controlling this carbonation step within the abovepH range, optimum precipitation of organic impurities occurs in thesubsequent causticization step. Although there may be some precipitationof hemicellulose during this carbonation step, the precipitation is farfrom complete and is difficult to separate from the liquor at thispoint. Lowering the pH substantially below 7.5 tends to increase theprecipitate but does not enhance its separation from the liquor. Whenthe pH is substantially more than 10.5,

insufficient carbonation occurs and subsequent inadequate precipitationof impurities in the subsequent causticizing step results.

Increasing the temperature during this controlled carbona' tion stepdoes not appear to enhance the separation.

Following carbonation, the spent liquor is treated with an excess oflime, sufficient to precipitate the bulk of impurities as carbonates andconvert the carbonated liquor to caustic soda. The quantity of limeadded is dependent upon the concentration of carbonates. To insurecomplete precipitation a portion of the supematent may be treated with alittle lime. If precipitation occurs, more lime is added to the bulkuntil no further precipitation is observed. Although this step may becarried out at room temperature, elevated temperatures of up to C. aresometimes beneficial to insure complete reaction and are preferred.Temperatures of over about 90 C, however, increase the cost ofprocessing without any subsequent improvement to the process.

After the lime treatment, the caustic soda liquor is separated from theprecipitated muds, by conventional separation means such as decanting orby filtration. The recovered caustic soda may thereafter be fortifiedwith strong caustic, or evaporated to a concentration usable for futurepulp purification, or for other uses. The lime muds are further washedto recover any additional caustic soda and then the muds may be burnedin a conventional manner to yield primarily water, carbon dioxide andburned lime. Both the carbon dioxide and lime may be reusable for aboveprocessing of caustic spent liquor.

Referring to the attached Flow Sheet, one embodiment of the presentinvention may be described as follows:

The caustic spent liquor containing hemicellulose is collected inStorage Tank 1 and passed through a Carbonating Tower 2 in which thecarbon dioxide flows countercurrent until sufficient carbonation occursto reduce the pH of the spent liquor from over 12 to about 7.5-10.5, andthe thus carbonated liquor is stored in a Liquor Storage Tank 3. It isthereafter transferred to a Causticizing Tank 4 and treated with anexcess of lime, agitating during the treatment so as to insure completeconversion of the bulk of sodium carbonate runm to sodium hydroxide. Thecausticized liquor is allowed to settle in tank 5 until the precipitatedcarbonates settle. The supematent is then decanted and transferred toStorage Tank 6 where it is stored for future use. The lime muds, afterwashing with water to remove residual regenerated caustic soda, aretransferred to the Kiln 7 and kilned, thus generating carbon dioxide foruse in the Carbonating Tower 2 and calcium oxide which is stored in aLime Slaker 8, for reuse in the above causticizing step.

The inventive process may be carried out at room temperature andatmospheric pressure. Temperatures may be increased to insure completereaction and to increase yields, but they need never be so high as thoseof the prior art methods for removing organic materials from spentliquors.

The following examples are included to illustrate specific embodimentsof this invention and are not intended to limit the scope thereof:

EXAMPLE I One liter of a spent caustic waste liquor containing about 80grams of sodium hydroxide, and 1.03 percent hemicellulose, obtainedafter purification of wood pulp in a conventional procedure and having apH of over 12, was first partially carbonated until a pH of 8.5 wasattained. To this was added gradually, 80 grams of calcium hydroxide (anexcess) to ensure not only conversion of sodium carbonate to causticsoda but maximum precipitation of hemicellulose. The mixture wasfiltered. The filtrate was found to contain 8.0 percent caustic soda andreduced in organic content to 0.23 percent, as cellulose.

EXAMPLE ll Another portion of waste liquor was treated in a manneridentical with that of example 1 except that carbonation pH value was9.7 and resultant cellulose residual in the regenerated alkali was 0.4percent.

Further tests indicated conclusively that neither carbonation norcausticizing techniques alone, even with filtration, produced a filtratewith substantial reduction in organic content.

l claim:

1. The process for recovering purified caustic soda solution from spentliquor obtained from alpha-cellulose purification having a pH of over l2and containing hemicellulose by the steps comprising:

a. carbonating said spent liquor with a carbon dioxide containing gasselected from the group consisting of carbon dioxide, flue gas and kilngas in an amount sufficient to reduce to pH to between 75 and [0.5,

b. causticizing said carbonated spent liquor with lime in an amountsufficient to precipitate impurities therefrom and convert saidcarbonated liquor to caustic soda, and

c. separating the purified caustic soda solution from the precipitate,

said controlled carbonation and causticization being conducted attemperatures of about room temperature to about C. and at substantiallyatmospheric pressures.

2. A process as in claim 1 wherein the gas in step (a) is flue gas.

3. A process as in claim 1 wherein under step (a) the pH is between9.0l0.0.

4. A process as in claim 1 wherein under step (a) the spent liquor ispassed through a Carbonating tower in which the carbon dioxidecontaining gas countercurrently flows.

5. A process as in claim 1 wherein under step (a) the carbon dioxidecontaining gas is bubbled through the spent liquor.

6. A process as in claim 1 wherein under step (b) the temperature isabout 90 C.

7. A process as in claim I wherein under step (a) the temperature isabout room temperature.

2. A process as in claim 1 wherein the gas in step (a) is flue gas.
 3. Aprocess as in claim 1 wherein under step (a) the pH is between 9.0-10.0.4. A process as in claim 1 wherein under step (a) the spent liquor ispassed through a carbonating tower in which the carbon dioxidecontaining gas countercurrently flows.
 5. A process as in claim 1wherein under step (a) the carbon dioxide containing gas is bubbledthrough the spent liquor.
 6. A process as in claim 1 wherein under step(b) the temperature is about 90* C.
 7. A process as in claim 1 whereinunder step (a) the temperature is about room temperature.